Importance of keeping meshes low-poly
It's completely unimportant, until you hit some limitation. Once you do, you know where it'll hurt, and possibly how to solve it.

A high-poly model can be used to bake bump maps, shadow maps, etc. which can be used to give a low-poly model a high-poly look. So even though you may not use the high-poly version in the end, it still is useful.